Flexible Constructor Bodies in Java 25 (JEP 513) Made Simple

September 19, 2025 - By Ashish Kumar

What Is JEP 513?

Java 25 introduces Flexible Constructor Bodies (JEP 513), a feature that makes constructors more flexible and easier to write. With this feature, you can now write code before calling super() or this() inside a constructor.

This means you can:

Validate inputs before passing them to the parent class.
Initialize fields before the parent constructor runs.
Avoid using static helper methods for simple tasks.

This change makes your code cleaner and safer, especially when dealing with validation or initialization logic.

Why This Feature Was Needed

Before Java 25, the first line of a constructor had to be super(...) or this(...).

This caused some problems:

You couldn’t validate input before calling the parent constructor.
You often needed static helper methods to handle validation.
Fields in the subclass couldn’t be initialized before the parent constructor ran.

👉 In short: constructors felt rigid and awkward.

What Changed in Java 25

Now, in Java 25, you can:

Write statements before super(...) or this(...).
Validate input directly inside the constructor.
Assign values to fields before calling the parent constructor.

⚠️ Rules to remember:

You cannot use this, call instance methods, or read instance fields before super(...).
You can only assign values to fields.

Example 1: Validation Before Super

Before Java 25 (old way):

public class Child extends Parent {
    private final String normalizedName;

    public Child(String name) {
        super(validateAndNormalize(name)); // must be first
        this.normalizedName = validateAndNormalize(name);
    }

    private static String validateAndNormalize(String s) {
        if (s == null || s.isBlank()) {
            throw new IllegalArgumentException("Name required");
        }
        return s.trim().toUpperCase();
    }
}

Problems: Duplicate validation and a static helper method needed.

After Java 25 (New Way):

public class Child extends Parent {
    private final String normalizedName;

    public Child(String name) {
        if (name == null || name.isBlank()) {
            throw new IllegalArgumentException("Name required");
        }
        String norm = name.trim().toUpperCase();
        this.normalizedName = norm;  // field initialized
        super(norm);                 // parent constructor called
    }
}

✅ Cleaner, no duplicate validation, no static helper required.

👉In the new approach, you can validate the input and initialize the field before calling super(), making the code more readable and less error-prone.

Example 2: Fixing a Classic Bug

Sometimes the parent constructor calls a method overridden by the child. In older versions, subclass fields might not be ready yet. causing bugs.

Old behavior:

class Parent {
    Parent() { setup(); }
    void setup() { System.out.println("Parent setup"); }
}

class Sub extends Parent {
    private int value;

    Sub(int v) {
        this.value = v; // runs AFTER super()
    }

    @Override
    void setup() {
        System.out.println("value = " + value);
    }
}

Output: value = 0 (because value wasn’t set before setup() ran).

New Java 25 behavior:

class Sub extends Parent {
    private int value;

    Sub(int v) {
        this.value = v;  // initialize field first
        super();         // parent constructor runs safely
    }
}

Output: value = 5 (or whatever you passed).

Key Points to Remember

No this or instance method calls before super().
Field initialization is allowed before super().
Helps you write cleaner and safer constructors.
Eliminates the need for static helper methods for validation or initialization.

When to Use Flexible Constructor Bodies

To validate inputs naturally inside constructors.
To initialize subclass fields before parent methods run.
To prepare values once and reuse them in both subclass and parent constructors.

🚀 Why It Matters

Cleaner code: You can write validation and initialization logic directly in the constructor.
Safer initialization: Prevents issues where the parent constructor might call overridden methods before the subclass is fully initialized.
No need for static helpers: Eliminates the need for static methods to handle validation or initialization.

Key Takeaways

Java 25 makes constructors more flexible.
You can now write logic before super(...).
Great for validation, field setup, and cleaner code.
Just remember: no this or method calls before super(...).